The mermaid boy is essentially composed of water, as opposed to solid matter. Thus, he has to live within a barrier, since he doesn't know how to maintain his form in the real world. Also, his mermaid powers allow him to construct his own spaces (that cannot be seen by normal humans), to let him use water as a medium with which to see everything and anything, etc. However, they call him a "spirit" or "ghost" because normal humans (aka "the monkeys") cannot perceive him at all. He exists on a different plane of existence.
The mermaid boy is essentially composed of water, as opposed to solid matter. Thus, he has to live within a barrier, since he doesn't know how to maintain his form in the real world. Also, his mermaid powers allow him to construct his own spaces (that cannot be seen by normal humans), to let him use water as a medium with which to see everything and anything, etc. However, they call him a "spirit" or "ghost" because normal humans (aka "the monkeys") cannot perceive him at all. He exists on a different plane of existence.